fix(Message): include MessageEmbed type

[almostSouji]

Please describe the changes this PR makes and why it should be merged:

MessageEditOptions#embed allows a MessageEmbed structure to be passed. This is currently not reflected in documentation or typings which only allow the raw embed data (type: MessageEmbedOptions).

This PR adds the typings and documentation to rectify this.

import { MessageEmbed, MessageEditOptions } from 'discord.js';
const data: MessageEditOptions = {};
data.embed = new MessageEmbed();

(property) MessageEditOptions.embed?: MessageEmbedOptions | null | undefined
Type 'MessageEmbed' is not assignable to type 'MessageEmbedOptions'.
  Types of property 'timestamp' are incompatible.
    Type 'number | null' is not assignable to type 'number | Date | undefined'.
      Type 'null' is not assignable to type 'number | Date | undefined'.ts(2322)

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.